This invention relates generally to an electronic blasting system and is particularly concerned with limiting the effects of undesirable leakage currents which may occur in the system.
As used herein “leakage current” includes current flow in a blasting system that does not directly contribute to the effective operation of electronic detonators in the system.
An electronic blasting system typically rakes use of a plurality of electronic detonators. Each detonator may include one or more capacitors which are used to ensure autonomous operation of the detonator if connecting wires to the detonator are broken, for example due to the effect of a blast at an adjacent borehole which may manifest itself before the detonator is to initiate.
The energy density requirement imposed by current consumption of an electronic detonator, the energy required to initiate a fuse head and space constraints side a detonator tube normally suggest the use of tantalum capacitors over other capacitor technologies. A tantalum capacitor has a low internal leakage current and is suitable for use in a blasting system which may call for hundreds, or even thousands, of electronic detonators to be used in a single blast.
Although a tantalum capacitor performs well in this type of application it can suffer from a failure mode that could result in the capacitor short-circuiting. This type of event can be catastrophic in a blasting system in which electronic detonators are connected in parallel to one another and are powered through a shared wire bus connected to a control device which may be some distance from a blast bench.
A short circuit in a capacitor, which is inside a detonator, results in a high current consumption and severe voltage starvation on a wire bus to which the detonator is connected. This can reduce the voltage available for blasting which, in turn, can cause a misfire in the affected detonator and which can also cause other detonators in the blasting system to misfire. Alternatively a significant time delay may be incurred while the faulty detonator is being identified and remedial action is taken.
The aforementioned problem is exacerbated if this type of failure only occurs once a supply voltage is raised to a level which is sufficiently high to supply blasting energy. The voltage increase is typically done only once mining equipment and personnel hare been evacuated from a blasting area and production would thus be brought to a standstill during this time. A delay of this type can result in significant financial losses.
There are other failures which can be detrimental to a parallel blasting circuit. For example damage to the insulation of wires, that typically occurs when a detonator is loaded into a blast hole, can cause current leakage or a short-circuit. A similar adverse effect can be produced by the ingress of a fluid into a detonator due to poor sealing between a crimp plug and a detonator tube, and similar factors.
A further concern is that a current leakage problem may not be apparent immediately when a detonator is loaded into a blast hole but may only appear after some time, possibly as wire to the detonator stretches during hole slumping, or due to a slow ingress of fluid into the detonator over time.
A typical blasting system can tolerate leakage currents of the order of tens of milliamps before voltage starvation occurs at which stage detonators may misfire due to insufficient voltage levels.
One approach adopted to identify where a leakage current or short circuit problem occurs in a blast system is to query the detonators in the system, electronically, to establish whether sufficient voltage is available. This approach allows a problem blast hole to be identified. For example, a detonator in the blast hole can measure an applied voltage and then respond to an interrogating module indicating whether the voltage is adequate or too low. Alternatively, an indication of the position of a problem can be obtained if one or more detonators do not respond at all for example if insufficient voltage is available or if a fault is present.
The location of a single short circuit in a blast system can sometimes be identified by measuring the resistance between the bus wires. This approach works if the resistance per unit length of the bus wire is known and if there are no significant contributors current flow through the bus wire.
In some systems detonators are connected sequentially to a two-wire bus by means of electronic circuits each of which, typically, is housed in a connector located on surface adjacent a borehole. This sequential connection methodology can allow for accurate determination of the location of a leakage or short circuit problem as is contemplate in U.S. Pat. No. 8,646,387.
An alternative solution is presented in US patent application No. 2013/0036931. This application describes opening a switch in reply to a signal from a detonator in response to an event, thus potentially disconnecting a remaining chain of detonators from the wire bus. However, the disconnection of an entire chain of detonators is not helpful if the cause of the leakage problem lies in a single borehole.
U.S. Pat. No. 7,911,760 is similarly limited in that a remain chain of detonators is controlled via an actuator on a wire bus.
U.S. patent application Ser. No. 13/582,688 discloses the use of a resistor in a detonator connector. A current through the resistor is sensed by monitoring the voltage, over the resistor, which is used to switch the gate of a FET. The resistor is thus used as a sensing element and not for current limiting purposes.
Although it is possible to modify systems of the aforementioned kind to isolate problem holes, the electronics required are relatively complex and can be expensive.
The effect of leakage current in a blasting system can be limited by decreasing the resistance of the bus wire so that the voltage drop across the bus wire due to a leakage current is limited. This approach requires a thicker bus wire, or additional bus wire. Alternatively each end of the detonator bus wire connected to a control unit so that the bus wire can be driven from both ends. These methods can however produce additional delays, particularly if problems are experienced only when a blasting voltage is applied to the bus wire.
It is also possible to raise the blast voltage to overcome losses due to a problem near an end of the bus wire. This, however, can expose some detonators to a voltage which exceeds a rated operating voltage.
An object of the invention is to provide an inexpensive and easy to implement mechanism for limiting, at least to some extent, the impact of problems due to leakage currents.